Jeff Gamso has a fascinating post about the probabilities behind a DNA match.
If you do felony criminal law (from either side of the aisle) and I tell you the number is 6.17 quadrillion, you probably assume that I’m talking about DNA.
The number will reflect just how unlikely it is that the DNA in the whatever left at the scene could have come from anyone other than the defendant. There are four things you need to know about that number.
If you want to know what those four things are, read Jeff’s post. I want to talk about something else. It’s that number, 6.17 quadrillion. There’s an important proviso that’s missing, although Jeff touches on it at the end of his post:
Next time you’re on a bus or a plane or a train or in a restaurant or movie theater or anywhere where there are a bunch of people, look around.
You never know when your not-twin, the one whose DNA profile is the same as yours, might be in the crowd. Despite the odds of 1 in 6.17 quadrillion. Hell, it might be one of the jurors.
If you pick a person at random off the street — or one of the random readers of this blog — the odds of someone else having that person’s DNA are not 1 in 6.17 quadrillion. They’re only about 1 in 500. That’s because about 1 person in 500 was born as one of a pair of identical twins.
Identical twins occur when a single zygote in the mother’s womb splits into two separate zygotes which develop into two separate individuals. Since all the cells in the zygotes trace back to a single zygote and therefore a single fertilized egg, they have essentially the same DNA.
All of this makes me wonder why this problem doesn’t come up more often in news stories about trials. If a criminal defendant has an identical twin, claiming he committed the crime would explain the DNA evidence. As a bonus, it would also explain things like photo arrays, line-ups, and lots of other eyewitness testimony. I guess a defendant might be reluctant to accuse his closest brother of a crime, but I would think that a pair of identical twins creates enough reasonable doubt to protect them both.
I’ve heard that claiming someone else commited the crime is sometimes called the SODDI defense, which stands for Some Other Dude Did It. I’d think we’d occasionally hear about the METDI defense: My Evil Twin Did It.